Anyone who has ever driven a vehicle towing a trailer is familiar with the problem of backing the vehicle up until the towing ball aligns approximately with the socket on the tongue of the towed vehicle (trailer).
The ball mounted on the towing vehicle is not visible from the driver's seat, so he can only image where the ball is as he backs up toward the tongue of the trailer. As he comes close enough to the trailer, the socket of the trailer also becomes invisible behind the towing vehicle, so that he must imagine where both the ball and the socket are.
Naturally, when there is another person around to give verbal and hand signals, it becomes somewhat easier for the driver to align the ball in the socket. Even this is sometimes tricky, inasmuch as in addition to the fact that both ball and socket are or will become invisible before they are aligned, movement and motions are also reversed in the rearview mirror, and the vehicle motion itself is reversed because the steering wheel is on the front. When the driver turns to the left, the front of the vehicle moves to the right and the rear of the vehicle moves, with the ball, to the left, but it looks as though it were moving to the right in the rearview mirror.
With so many confusing signals to the driver, it is a real trick to properly align the ball and socket. Although an experienced driver may make the proper alignment the first time, an inexperienced driver will often need to stop the vehicle, get out and check the alignment, get back in and pull forward and try backing up again, to move the ball a foot or more to the left or right, or further rearwardly.
Depending on the weight of the trailer, the proper alignment with the towing vehicle can be critical. If the trailer tongue is extremely heavy, it will be necessary for the driver to position the ball immediately beneath the socket, because virtually no tugging of the trailer will be possible. It will have to be jacked down directly onto the ball. Devices have been created which will release some of this pressure and permit the trailer tongue to be rocked back and forth somewhat in its elevated position, although a perfect alignment in the first place would clearly be superior.
Additionally, complicated schemes have been developed utilizing a reel-in guide wire attached between the ball and socket with alarms indicating whether the vehicle is too forward left or right, and when it finally aligns. Other devices include "scoops," that fan out from the socket, which in effect provide a larger target area for the ball and guide it into place beneath the socket.
These types of devices and more lend credence to the fact that it is recognized in the world at large that backing up towing vehicles to align a ball with a trailer socket is a major nuisance.